One of the major limitations in measuring displacement with eddy-current proximity probes is their sensitivity to temperature which affects both the zero reading and the change (or span) of signal relative to the in target proximity. To reduce the effect of temperature on the zero value, one approach is to use two matched proximity probes, one positioned near a fixed metallic target, the other opposite the metallic surface whose displacement is to measured. Each probe is then part of separate oscillator-detector circuits whose signals are subtracted from each other. While this approach is an improvement, it still suffers from uncertainties due to long term drifts in the different circuit components, as well as from the need to carefully match circuit characteristics over the temperature and voltage changes of the particular environment.
Probe sensitivity can be greatly increased by near resonance capacitive tuning of the probe inductance. The tuning effect, however, is incompatible with prior art circuitry used to compensate for thermal effects, and will augment the resulting errors.